Purine nucleotides are well established as extracellular signaling molecules. P2X receptors are ATP-gated cation channels that mediate fast excitatory transmission, e.g., in diverse regions of the brain and spinal cord. The P2X7 subtype has the unusual property of changing its ion selectivity during prolonged exposure to ATP, which results in progressive dilation of the channel pore and the development of permeability to molecules as large as 900 Da. The P2X7 receptor was originally described in cells of hematopoietic origin, including macrophages, microglia, and certain lymphocytes, and mediates the influx of Ca2+ and Na+ ions, as well as the release of proinflammatory cytokines. P2X7 receptors may affect neuronal cell death through their ability to regulate the processing and release of interleukin-1β, a key mediator in neurodegeneration, chronic inflammation, and chronic pain. Activation of P2X7 receptors provides an inflammatory stimulus, and P2X7 receptor-deficient mice have substantially attenuated inflammatory responses, including models of neuropathic and chronic inflammatory pain. Moreover, P2X7 receptor activity, by regulating the release of proinflammatory cytokines, may be involved in the pathophysiology of depression. The P2X7 receptor may thus represent a critical communication link between the nervous and immune systems (Skaper et al. 2010 FASEB J. 24:337-345). The localisation of the P2X7 receptor to key cells of the immune system, coupled with its ability to release important inflammatory mediators from these cells suggests a potential role of P2X7 receptor antagonists in the treatment of a wide range of diseases including pain and neurodegenerative disorders, while providing a target for therapeutic exploitation.
In cancer where apoptotic cell death is an important mechanism of disease, P2X7 with its direct effect in apoptosis plays a significant role as it was shown in skin cancers and uterine epithelial cancers compared to normal tissues. Perhaps P2X7 will be of future use as biomarker to distinct normal from cancer uterine epithelial tissues.
Early apoptotic cell death to the retina in diabetes in rodent models has been linked to P2X7 activation in that part of the eye, suggesting a possible connection to diabetic microvascular injury.
It has been reported that P2X7 receptor polymorphisms may be linked to hypertension in a family based quantitative genetic association study, with a strong association of single nucleotide polymorphism rs591874 in the first intron of P2X7 and nocturnal diastolic blood pressure. P2X7 receptors are expressed in cells of the cardiovascular system and drugs affecting this signaling system may provide new therapies in hypertension and prevention of thrombotic events.
Expression of P2X7 receptors in healthy kidney is very little if any. In contrast, expression of P2X7 is increased in diseased renal tissue and immunohistochemistry of the glomeruli of two rodent models of kidney disease has shown that the predominant expression is in podocytes, endothelial and mesangial cells. A potential role for P2X7 receptors has been described for polycystic kidney disease and renal fibrosis.
Since ATP plays key roles in neurotransmission and neuromodulation, purine receptor subfamilies, including P2X7, have been involved in various pathological conditions. This pathophysiology of central nervous system (CNS) disorders includes brain trauma, ischemia, neurodegenerative and neuropsychiatric diseases. When injury happens, large amounts of ATP are released in the extracellular environment which are important for triggering cellular responses to trauma. In this situation, expression levels of P2X4 and P2X7 changes which might stimulate the migration and chemotaxis of resting microglia to the site of damage. P2X7 plays an important role in controlling microglia proliferation and death.
Cerebral ischemia can produce and exacerbate problems to the CNS which include stroke and it is possible that the P2X7 receptor which is expressed on microglia, is involved in cortical damage as a consequence of glucose/oxygen deprivation.
Neuroinflammation plays a major role in the pathogenesis of a number of neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease. Although the precise mechanism is obscure, dysregulation of the signaling transduction pathway in microglia may enhance inflammation, leading to synaptic dysfunction and ultimately to neuronal cell death. The expression and function of the P2X7 receptor is significantly up-regulated in the post-mortem brain of Alzheimer's disease patients and various neurodegenerative disease animal models. This supports the role of the P2X7R pathway in the progression of neurodegeneration. Blocking P2X7R using brilliant blue G, a P2X7R antagonist that can cross the blood-brain barrier, has been shown to result in the amelioration of neuropathology in various animal models. Synaptic alterations and increased susceptibility to neuronal death are known contributors to Huntington's disease (HD) symptomatology. Decreased metabolism has long been associated with HD. Recent findings have demonstrated reduced neuronal apoptosis in Caenorhabditis elegons and Drosophila models of HD by drugs that diminish ATP production. Extracellular ATP has been reported to elicit neuronal death through stimulation of P2X7 receptors and hence, alteration in P2X7-mediated calcium permeability may contribute to HD synaptic dysfunction and increased neuronal apoptosis. Using mouse and cellular models of HD, increased P2X7-receptor level and altered P2X7-mediated calcium permeability in somata and terminals of HD neurons has been demonstrated and in vivo administration of the P2X7-antagonist Brilliant Blue-G (BBG) to HD mice prevented neuronal apoptosis and attenuated body weight loss and motor-coordination deficits.
Taken together, these results raise the possibility for the P2X7R signaling pathway as therapeutic target for treating various neurodegenerative diseases including AD and HD.
Multiple sclerosis (MS) is an immunogenic, relapsing, chronic inflammatory disease. There is a huge potential for biologics as therapeutics for MS. The first generation of cytokines (IFN-γ-1a, IFN-γ-1b) and antibodies (against CD52, CD49d, CD25, CD20) yielded both promising and disappointing results in the clinic, but it is clear that there still is high medical need for therapeutics that relieve inflammation by targeting lymphocytes via novel mechanisms of action or that target chronically activated microglia and macrophages.
Although well validated, ion channels represent an underexploited target class for the treatment of MS. Development of drugs against ion channels has been hampered because small molecule inhibitors in general lack specificity and affect relatives of the target and even unrelated proteins.
P2X7 receptor is expressed on myeloid cells as well as on CNS glial cells, and P2X7 activation has been shown to increase both glial and T-cell activation. These properties suggest a role in the development of autoimmune diseases including MS. P2X7 deficiency in an animal model of MS, experimental autoimmune encephalomyelitis (EAE), was shown to result in compensatory changes leading to increased T-cell cytokine production, and activated T-cells were detected in the brains of P2X7 null mice with no clinical signs. The greatly reduced incidence of disease suggested that an initiating event is absent in these mice and points to a role for astroglial P2X7 in development of EAE disease.
Matute et al (J. Neuroscience 2007: 27(35):9525-9533) have shown that Enhanced ATP signaling in vitro and in vivo leads to oligodendrocyte death via P2X7 receptor-mediated Ca2+ toxicity and that P2X7 receptors mediate tissue damage underlying the neurological deficits associated with well-established models of MS. In turn, the increased expression of P2X7 receptors in axon tracts before lesions are formed in MS suggests that this feature constitutes a risk factor associated with newly forming lesions in this disease. Blockade of ATP P2X7 receptors therefore has potent neuroprotective properties, suggesting that this mechanism could be exploited to halt the progression of tissue damage MS.
P2X7 specific polyclonal and monoclonal antibodies have been described (Adriouch et al., 2005 “Probing the expression and function of the P2X7 purinoceptor with antibodies raised by genetic immunization” Cell Immunol 236:72-77; Seman et al. 2003. NAD-induced T cell death: ADP-ribosylation of cell surface proteins by ART2 activates the cytolytic P2X7 purinoceptor. Immunity 19:571-582). It was indicated that these antibodies are useful tools for further characterization of the structure and function of P2X7. US patent application No 20100173799 describes Nanobodies that bind P2X7.